Here's one of the scenes that really hammered home for me what subtext really is.  It's one of the coolest things about acting for me.  What really gets me about this scene is Robin Williams' performance.  It really is a great scene.  Take a look:

If you think about it, all he's really doing is listening and letting Sally Field talk.  And even though Field is the one that's talking, the scene really isn't about her at all.  It's about Robin and his reaction to what she says.

If you haven't seen the film, I'll give you a brief rundown.  Robin Williams and Sally Field are a recently divorced couple and Williams dresses up as a housekeeper in order to spend more time with his three kids.

The great thing about this scene is that Williams is constantly juggling the two characters throughout the conversation.  It's this great plot device where he gets to secretly hear what his ex-wife really thinks of him, and it's when this information really gets to him that he drops the Mrs. Doubtfire mannerisms and gives us little windows into how the heart-broken ex-husband really feels about what he's hearing.

When Williams says the line "Really?" at around frame 150, he gives a really big smile that you can tell isn't something that the calm, well-mannered, restrained Mrs. Doubtfire would give us.  He's giving us that expression as the husband and really letting us know that he's glad that he made his ex-wife happy.

The next line he gives is almost the total opposite of the first.  It's almost like he flips a switch.  Here he's almost completely his Mrs. Doubtfire persona, he changes his voice, puts more emphasis on her accent, sits up in his chair a little more.  He's more controlled and less affected personally by the conversation he's having.

As the scene progresses, he's letting more and more of himself shine through to the point where he becomes less and less Mrs. Doubtfire and more the heart-broken husband eavesdropping on his ex-wife.

By the time he says "Really?" again around frame 1250, he has a totally different reaction from the first line in the scene.  On paper, he could really have played both of those lines the same way, but instead he gives the line subtext by showing us that he's disappointed in himself and his short-comings as a husband.  Here he's really letting what Field is saying get to him.  Even though he's still in Mrs. Doubtfire makeup, he still can't help but be himself.

The real tear-jerker shot for me is the silent reaction he gives around 1450.  He does this great thing that people seem to do right when they're getting ready to cry.  He swallows.  It's not a normal swallow either, it's sort of a tense swallow where his jaw clenches up a little like he's fighting back tears.  I could be over-analyzing it, but that sort of thing really gets to me.

Around 1850 he's in full-on husband mode and completely forgets that he's in disguise.  He's about to say "You never put it that way before."  But he catches himself mid-sentence, flips the switch, and phrases it differently.  This time his voice snaps back to Mrs. Doubtfire and he avoids revealing himself to her.

There's another great little moment where he's listening to her say "I used to think Daniel could do anything..." (around frame 2370).  The first part of the sentence makes him happy, and when she delivers the rest of the line "...except be serious." she sort of lets all the air out of his sails.  It's a really subtle change in facial expression.  He goes from having that slight smile, eyes squinted, cheeks lightly raised, to being totally deflated and listless within maybe 10 or 20 frames.  And all of this through prosthetic makeup!  It's really amazing to see him give such a subtle performance through a few centimeters of latex.  It's all mostly in his eyes!